The invention relates to a signal compressor for an audio apparatus, in particular a motor vehicle audio apparatus, which contains a volume control that is inserted in the audio signal path and is adjustable via a digital setting signal delivered by a digital controller.
With mass-produced articles, such as e.g., audio equipment for motor vehicles, which contain at least a radio along with a cassette unit and a CD unit, it is desirable to combine as many circuit parts as possible in an integrated circuit for the reduction of space and production costs. Integrated circuits can be equipped also with features that facilitate handling and improve the sound pattern.
While numerous individual components of an audio apparatus can be combined in an audio processor, there are still certain functions realized as before through discrete components or component groups that could not be designed as an integrated circuit.
The present invention is concerned with signal compression for the dynamic compression of audio signals. Depending on the nature of the memory medium, audio signals, in particular for instance with classic music, have a large dynamic range (e.g., up to about 85 dB). This large dynamic range reflects essentially the situation in a concert hall. In a piece of music, there are very soft passages and extremely loud passages. While the true reproduction of these very soft and very loud passages of a piece of music is desirable as such, the high dynamic range in case of motor vehicle audio equipment and other equipment operated in environments with high background noise level frequently results in undesirable effects. For example, soft passages almost cannot to be heard because of the high background noise level, while loud passages are felt as disturbing, particularly when they distract from another activity, e.g., the driving of a motor vehicle.
For this reason it is already known to provide an audio apparatus with a signal compressor, which reduces the very large dynamic range of the audio signal, e.g., from 85 dB to 60 dB. The effect of such a signal compressor consists in that the loudness is increased in very soft passages of a piece of music, while the loudness in very loud passages is decreased.
The applicant has already developed a signal compressor that is used together with an audio processor operating in digital fashion to the largest possible extent. For example, by means of a signal compressor connected upstream of the audio processor, the non-compressed output signal of a CD apparatus (compact disk apparatus) is fed to the signal compressor, compressed therein, and the compressed output signal is fed to the audio processor.
These previous signal compressors were designed as circuits operating in an analog manner. To obtain the desired signal compression of the analog signal, resistors and capacitors were used. Such components, which were usually employed as external components, in addition to integrated components, are contrary to mass production at low costs.
To convert the non-compressed signal to a compressed signal, typically an analog multiplication of the non-compressed analog signal by an appropriate factor is carried out.
What is disadvantageous with the known multiplier is, however, that the compressed signal obtained displays a strong distortion, in particular with input signals of high level.
Since the known signal compressor designed as a separate component displays a series of active signal processing stages, the signal to noise ratio (S/N ratio) of the signal deteriorates, particularly at low signal levels.
The invention is directed to providing a signal compressor for an audio apparatus in which the desired signal compression can be realized using a comparatively fewer number of components. In particular, space is saved by saving components of the entire audio apparatus, and the production is less expensive.
The invention achieves a reduction in the harmonic distortion and noise inevitably created with conventional signal compressors.
The invention provides a signal compressor of the type indicated at the outset, which has the following features:
a) the audio signal is branched off from the audio signal path upstream of the volume control and is processed by a signal processing circuit so as to form a tracking signal that is dependent on the intensity of the audio signal;
b) the tracking signal is compared in a comparator with a reference value;
c) depending on the comparison result, a counter designed as up/down counter is increased or decreased; and
d) the contents of the counter are linked with the digital setting signal before the setting signal is fed to the volume control.
A favorable prerequisite for the invention is an apparatus having a volume control that is adjusted by a digital setting signal. Such a volume control has already been developed by the applicant (DE-A-195 27 338). In this volume control the level of the analog audio signal at the input of the volume control is changed on the basis of a digital setting signal, such that when there is a change in volume, a soft or smooth signal transition without any xe2x80x9cclicking noisexe2x80x9d results. When the digital value of the setting signal changes (which can happen suddenly), the audio signal at the output of the volume control normally would perform an amplitude jump, which expresses itself as a xe2x80x9cclicking noisexe2x80x9d to a listener. The effect achieved by the measures described in detail in said DE-A-195 27 338 is that this signal transition at the output of the volume control takes place in a soft and smooth manner, so that the undesirable clicking noise is avoided.
The known volume control has a digital input for a digital setting signal. In the invention this digital setting signal is modified depending on the level of the audio input signal. The tracking signal dependent on the level of the audio signal upstream of the volume control is compared with a reference signal, and depending on the comparison result, the count of an up/down counter is changed. The contents of the counter then are linked in suitable manner with the digital setting signal.
The various linking types possible are described in more detail further below. One advantage of the invention is that very weak levels of the audio signal cause the volume control to change the signal towards a higher volume, while at high levels of the audio signal, the volume control performs a change in the audio signal in the sense of a volume reduction.
As will be described in more detail below, the components of the signal compressor can be designed completely as an integrated circuit. Due to the fact that use is made in part of already existing components in the audio processor, considerable space is saved for the circuit on the circuit board as compared to earlier designs, i.e., a signal compression with the aid of an independent signal compressor.
Because no multiplication of analog signals is required, the audio signal changed in intensity has little, if any, detectable distortion. As there are no additional components necessary in the signal path proper, the signal to noise ratio is not affected.
The aforementioned linkage of the digital setting signal with the contents of the counter can be accomplished in different ways. In one embodiment, an adder is provided that receives on its first input the digital setting signal and has a second input coupled with the output of the counter, the counter having its count raised by the output signal of a comparator when the tracking signal corresponds to a reduced level of the audio signal.
To elucidate the operation of this specific embodiment comprising the adder, some numerical examples shall be used: When the digital setting signal for the volume control has 8 bits, there are theoretically 256 volume levels possible. It shall be assumed that the volume control is set to the level xe2x80x9c100xe2x80x9d, i.e. that the digital setting signal is the binary representation of the decimal value xe2x80x9c100xe2x80x9d. When the up/down counter has six digits, the contents thereof can represent any value between 0 and +63. When the audio signal has a medium level, the contents of the counter correspond, e.g., for example, to the value xe2x80x9c30xe2x80x9d. By addition to the above-mentioned value xe2x80x9c100xe2x80x9d, the volume control then is actually set to a value of xe2x80x9c130xe2x80x9d.
When the audio signal has a very weak level, a correspondingly lower tracking signal is formed, which e.g., is lower than the reference signal, whereby the output signal of the comparator increases the contents of the counter in corresponding manner. The extent of the increase of the counter depends on how low the level of the audio signal decreases. At extremely low levels of the audio signal, the counter is increased in correspondingly great manner, at maximum to the value xe2x80x9c63xe2x80x9d. This value xe2x80x9c63xe2x80x9d is added to the aforementioned value of xe2x80x9c100xe2x80x9d, so that the volume control actually sets a volume corresponding to the value xe2x80x9c163xe2x80x9d.
In case of very high levels of the audio signal, the counter is operated in such a manner that its count becomes lower. As a result, the volume control then delivers a signal corresponding to the set value of xe2x80x9c100xe2x80x9d in the extreme case, i.e., the value of the digital setting signal.
The foregoing can also be achieved when, in a further embodiment of the invention, a subtractor is used instead of the adder. In this case, the count of the counter is decreased in response to a very low level of the audio signal. The subtraction carried out by the subtractor, in case of weak levels of the audio signal, results in an only minor reduction of the numerical value corresponding to the digital setting signal, so that weak audio signals are relatively increased. In contrast, with strong audio signals at the input of the volume control, a correspondingly high value is subtracted from the digital setting signal, attenuating the high volume.
Other modifications and embodiments are possible for linking the digital setting signal with the output signal of the counter. For example, a digital multiplication can be carried out. At medium signal levels of the audio signal, the value of the digital setting signal then would be left more or less unchanged (multiplication factor xe2x80x9c1xe2x80x9d); whereas at low signal levels of the audio signal, an increase in volume would be obtained (multiplication factor between 1.01 and 1.15). At very high audio signal levels, the multiplication would lead to a reduction in volume (multiplication factor e.g., between 0.85 and 0.99).
To obtain a xe2x80x9cslidingxe2x80x9d compression of the input audio signal in the entire level range, the invention provides a comparator designed for analog signal processing with one input connected via a D/A converter to the output of the counter and another input connected to the output of the signal processing circuit. The reference signal compared by the comparator to the tracking signal thus corresponds to the current count of the up/down counter. With an extremely weak audio signal, for instance, the tracking signal is higher than the reference signal, as a result of which the count is increased (if an adder is connected downstream thereof). This also increases the value of the reference signal. Subsequently, a further increase in count takes place as long as the tracking signal is higher than the reference signal. Due to the step-like increase of the reference signal, a condition is finally reached in which the tracking signal is lower than the reference signal, so that the count then is no longer increased.
Instead of converting the count of the digital counter to an analog signal and then feeding this analog signal as a reference signal to the comparator along with the analog tracking signal, another preferred embodiment of the invention provides a comparator designed as a digital signal comparator with one input connected via an A/D converter to the output of the signal processing circuit and another input connected to the output of the counter. This circuit with the comparator operating in digital manner delivers essentially the same signals for increasing or decreasing the count of the up/down counter. The above-mentioned signals that are fed to the up/down counter are the signals that determine whether the count is increased or decreased. The counting operation depends on the clock signal fed to the counter. The counter in this respect is designed such that, with different levels of xe2x80x9cupxe2x80x9d and xe2x80x9cdownxe2x80x9d signals coming from the comparator, a corresponding change of the count by xe2x80x9c1xe2x80x9d takes place, depending on the clock signal supplied. When the signals xe2x80x9cupxe2x80x9d and xe2x80x9cdownxe2x80x9d have the same levels, the clock signal effects no counting operation.
Compressing of the dynamic signal range with the aid of the signal compressing circuit according to the invention, of course, should take place in such a manner that the music does not appear strange because of unexpected volume changes. Suddenly occurring high volumes can be undesirable in particular for the driver of a motor vehicle. To avoid this and to obtain a smooth transition as possible in the compressed signal, the invention provides that the output of the comparator is coupled with a counting direction input of the counter and that the counter, depending on whether the audio signal intensity increases or decreases, is clocked with a clock signal having a first frequency and, respectively, with a clock signal having a second frequency that is lower than the first frequency.
The effect achieved thereby is that the level decrease with increasing audio signal is carried out in a shorter period of time than a correspondingly high level increase with decreasing audio signal. The level decrease with increasing audio signal is effected in a shorter period of time, in order to prevent amplitude clipping with strongly increasing audio signal. The level increase with decreasing audio signal is carried out more slowly in order to prevent a pumping effect in case of a strongly changing audio signal. In this way the output signal is smoothed.
The clock signal of the first, relatively high frequency takes care of a strong level rise, and clipping of the signal is prevented. The second, relatively low frequency, which is decisive at a decreasing level, smoothes the output signal, and in particular avoids a pumping effect that takes place with strongly changing signals.
The clock signal for clocking the counter is obtained by an oscillator forming, from a signal of relatively high frequency f1, by division or frequency division a clock signal with a frequency f2 corresponding to a fraction of the frequency f1. Whether the counter is fed with the signal of higher frequency or the signal of lower frequency is dependent on whether the level of the audio signal is increasing or decreasing at the particular time. This circuit for delivering the clock signals is not essential for the understanding of the present invention and shall not be elucidated here in more detail.
According to the invention, a tracking signal is formed of the branched audio signal, which is compared with a reference signal. A particular embodiment of the invention provides that the signal processing circuit used therefor has the following features:
a rectifier rectifying the audio signal and, if applicable, adding the separate signals for the left and the right channels;
a logarithmic amplifier connected to the output of the rectifier; and
a low pass filter at the output of the logarithmic amplifier.
The logarithmic amplifier serves to match the loudness change to the loudness sensation of the human ear. The low pass filter connected downstream of the amplifier smoothes the signal, so that it is suitable for a comparison with the (changing) reference signal.
The comparison between the tracking signal and the reference signal carried out in the comparator is rendered insensitive with respect to residual disturbances in the tracking signal in that a window comparator is used. Such window comparators are known. They deliver a corresponding comparison signal at their output when the signal (here the tracking signal) compared with the reference signal is outside a window, the center of which is formed through the reference signal.
A signal compressor according to the invention can also be employed for so-called xe2x80x9canti-clippingxe2x80x9d. Clipping is understood to be the effect that, due to too high dynamics or signal intensity, signals exceed level values corresponding to the positive and negative voltage supply values, and undergo amplitude limitation at these voltage supply values. It is usual to form output stages or output power stages with a device delivering a harmonic distortion signal indicating the harmonic distortion. This signal can be fed to the signal compressor according to the invention, by means of which the signal subject to the risk of clipping, depending on the type of the harmonic distortion signal, is decreased when approaching the clipping range and is increased again when this range is left again.
The signal compressor according to the invention can also be employed in connection with noise-dependent volume matching, for instance in a car radio. By means of a microphone, the background noise volume is ascertained and a resulting background noise signal is passed to the signal compressor in order to increase or decrease, or decompress or compress, respectively, the audio signal according to the intensity of the background noise.